A packet switch communication system includes a network of one or more routers connecting a plurality of users. A packet is the fundamental unit of transfer in the packet switch communication system. A user can be an individual user terminal or another network. A router is a switching device that receives packets containing data or control information on one port and, based on destination information contained within the packets, routes the packets out another port to their final destination, or to some intermediary destination(s). Conventional routers perform this switching function by evaluating header information contained within the packet in order to determine the proper output port for a particular packet.
As known, a communications network can be a public network, such as the Internet, in which data packets are passed between users over untrusted, i.e., non-secure communication links. Alternatively, various organizations, typically corporations, use what is known as an intranet communications network, accessible only by the organization's members, employees, or others having access authorization. Intranets typically connect one or more private servers, such as a local area network (LAN). The network configuration in a preferred embodiment of this invention can include a combination of public and private networks. For example, two or more LANs can be coupled together with individual terminals using a public network, such as the Internet. A network point that acts as an entrance to another network is known in the art as a gateway.
Conventional packet switched communication systems that include links between public and private networks typically include means to safeguard the private networks against intrusions through the gateway provided at the interface of the private and public networks. The means designed to prevent unauthorized access to or from a private are commonly known as firewalls, which can be implemented in both hardware and software, or a combination of both. Thus, a firewall is a device that can be coupled in-line between a public network and a private network for screening packets received from the public network.
Referring to FIG. 1, a conventional packet switch communication system 100 can include two (or more) private networks 102a and 102b coupled by a public network 104 for facilitating the communication between a plurality of user terminals 106. Each private network 102 can include one or more servers and a plurality of individual terminals. Each private network 102 can be an intranet, such as a LAN. Public network 104 can be the Internet, or other public network having untrusted links for linking packets between private networks 102a and 102b. In a preferred embodiment, at each gateway between a private network 102 and public network 104 there is a firewall 110.
The architecture of an illustrative prior art firewall is shown in FIG. 2a. The firewall 110 generally includes one or more public network links 120, one or more private network links 122, and memory controller 124 coupled to the network links by a PCI bus 125. Memory controller 124 is also coupled by a memory bus 129 to a memory (RAM) 126 and a firewall engine, implemented in a preferred embodiment as an ASIC 128. The firewall engine ASIC 128 performs packet screening prior to routing packets through to private network 102. The firewall engine ASIC 128 processes the packets to enforce an access control policy, screening the packets in accordance with one or more sets of rules. The rules are described in more detail below. A central processor (CPU) 134 is coupled to memory controller 124 by a CPU bus 132. CPU 134 oversees the memory transfer operations on all buses shown. Memory controller 124 is a bridge connecting CPU bus 132, memory bus 129, and PCI bus 125.
In operation, packets are received at public network link 120. Each packet is transferred on bus 125 to, and routed through, memory controller 124 and on to RAM 126 via memory bus 129. When firewall engine 128 is available, packets are fetched using memory bus 129 and processed by the firewall engine 128. After processing, the packet is returned to RAM 126 using memory bus 129. Finally the packet is retrieved by the memory controller 124 using memory bus 129, and routed to private network link 122. The screening rules implemented by the firewall engine 128 are typically searched in linear order, beginning with the internal rule memory. Certain aspects of the rule structure are described below.
As known in the art, a rule is a control policy for filtering incoming and outgoing packets. Rules specify actions to be applied as against certain packets. When a packet is received for processing through a rule search, the packet's IP header, TCP header, or UDP header may require inspecting. A rule will generally include, at a minimum, source/destination IP addresses, UDP/TCP source/destination ports and transport layer protocol. Additional criteria may be used by the rules as well.
Generally, the address information is used as matching criterion—in other words to match a rule, a packet must have come from a defined source IP address and its destination must be the defined destination IP address. The UDP/TCP source/destination port specifies what client or server process the packet originates from on the source machine. The firewall engine can be configured to permit or deny a packet based upon these port numbers. The rule may include a range of values or a specific value for a TCP/UDP port. The transport layer protocol specifies which protocol above the IP layer, such as TCP or UDP, the policy rule is to be enforced against.
The firewall engine described above essentially screens packets using an access control list (ACL), and may be referred to as an ACL engine. That is, it performs a simple comparison of various matching criteria of an incoming IP packet—typically source, destination, port and protocol—to each rule in a rule set in sequence. Based upon this comparison, an incoming IP packet is either allowed or denied. A data-flow chart for this firewall engine is shown in FIG. 5.
It will be appreciated that using a fixed set of rules can be restrictive in many practical applications. Therefore, it is desirable to provide a system and method capable of adding rules to the rule set of the firewall engine dynamically—that is, to extract from a sequence of packets information, such as the port number and IP address, and generate new rules using this information. However, generating these new rules dynamically would increase the complexity of the comparison and decrease the speed of the firewall engine. There is therefore a need in the art for a firewall engine which can generate rules dynamically, based upon information extracted from incoming packets, with a limited impact on the speed of the firewall engine.